1. Field of the Invention
The present invention relates to a method for manufacturing a toner and a toner for use in electrophotography.
2. Discussion of the Background
In electrophotography, electrostatic recording, electrostatic printing, etc., a developer is adhered to an image bearing member, such as an electrostatic latent image bearing member on which an electrostatic latent image is formed, and then transferred from the image bearing member onto a transfer medium such as a paper, and finally fixed on the paper. As the developer configured to develop the electrostatic latent image formed on the image bearing member, a two-component developer including a carrier and a toner and a one-component developer consisting essentially of a toner (e.g., magnetic toner and non-magnetic toner) are known.
As a dry toner for use in electrophotography, electrostatic recording, electrostatic printing, etc., a pulverized toner in which a toner binder such as a styrene resin and a polyester resin, a colorant, etc. are melt-kneaded and pulverized is widely used.
Recently, polymerized toners which are manufactured by polymerization methods such as suspension polymerization methods and emulsion polymerization aggregation methods are studied. Published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 07-152202 discloses a polymer dissolution suspension method. In this method, toner components are dispersed and/or dissolved in a volatile solvent such as an organic solvent having a low boiling point to prepare a toner component mixture liquid. The thus prepared mixture liquid is emulsified in an aqueous medium containing a dispersant to form droplets of the mixture liquid. Finally, the volatile solvent is removed from the droplets to prepare toner particles while contracting the volume of the droplets. Unlike suspension polymerization methods and emulsion polymerization aggregation methods, the polymer dissolution suspension method has an advantage that various kinds of resins can be used. For example, a polyester resin, which is useful for obtaining a full-color image having transparency and smoothness, can be used for this method.
Since the polymerized toners are prepared in an aqueous medium containing a dispersant, the dispersant tends to remain on the surface of the toner and deteriorates chargeability and environmental stability thereof. In order to remove the remaining dispersant, a large amount of water is needed. Thus, the polymerized methods are not necessarily satisfactory.
In attempting to solve the above problems, JP-A 2003-262976 discloses a toner manufacturing method in which microdroplets of fluid raw materials are formed using piezoelectric pulse and then dried to become toner particles. JP-A 2003-280236 discloses a toner manufacturing method in which microdroplets of fluid raw materials are formed using thermal expansion of the nozzle and then dried to become toner particles. JP-A 2003-262977 discloses a toner manufacturing method in which microdroplets of fluid raw materials are formed using an acoustic lens and then dried to become toner particles. These methods have a disadvantage that manufacturability of the toner is poor because the number of the droplets discharged per unit time is small. In addition, it is difficult to prevent each of the droplets from uniting with each other, resulting in broad particle diameter distribution of the resultant particles. Thus, these methods are also not necessarily satisfactory.
JP-As 2006-28432 and 2006-28433 have disclosed methods in which a dispersion liquid, in which toner components including a light curing resin and a thermal curing resin, respectively, are dispersed in a dispersion medium, is intermittently discharged from a nozzle to form droplets, and then the droplets are aggregated while the light or thermal curing resin is cured to stably form particles. However, these methods also have poor manufacturability and the resultant particles have a wide particle diameter distribution. In addition, the cured resins do not impart satisfactory fixability to the resultant particles.
These methods have a feature that the dispersion liquid directly contacts a vibration part. In this case, when the number of holes and that of the vibration part are same, the resultant particles have a narrow particle diameter distribution. But when there are a plurality of holes and a single vibration part, the size of the droplet depends on the distance between the hole and the vibration part. Therefore, different holes produce particles having different particle diameters.
The dry toner is typically fixed on a recording medium (such as paper) upon application of heat by directly contacting a heat roller or belt. When the temperature of the heat roller or belt is too high, an offset problem tends to be caused in that excessively melted toner is adhered to the surface of the heat roller or belt. In contrast, when the temperature of the heat roller or belt is too low, the toner cannot be sufficiently fused and fixed.
In terms of energy saving and downsizing of apparatuses, a need exists for a toner which minimizes hot offset (this property is hereinafter referred to as hot offset resistance) and which can be fixed at low temperatures (this property is hereinafter referred to as low-temperature fixability). The toner is also required to have a property such that the toner does not cause a blocking problem even when the toner is stored at the temperature inside an apparatus (this property is hereinafter referred to as thermostable preservability). In particular, full-color copiers and printers are required to produce images having glossiness and color-mixing property, and therefore polyester resins are widely used as a full-color toner binder because of having low melt-viscosity.
Since such a toner easily causes hot offset, a silicone oil is typically applied to a heat member in the full-color copiers and printers. In this case, the apparatus needs an oil tank and an oil applicator, and therefore the apparatus must be larger and complicated. There is another problem such that the oil applied to the heat member tends to adhere to copier papers and overhead projection (OHP) films, resulting in deterioration of the color tone of the produced images.
In attempting to solve these problems, a technique in which a release agent (such as wax) is added to a toner is proposed and widely used to prevent the toner from adhering to the heat roller without applying an oil thereto. Releasability of the toner greatly depends upon dispersing conditions of the wax in the toner. When the wax is compatible with the binder resin used, the toner has no releasability. When the wax is incompatible with the binder resin and forms domains thereof in the toner, the toner has releasability. In this case, when the domains are too large, the amount of the wax existing near the surface of the toner relatively increases. Thereby, the toner particles tend to aggregate, resulting in deterioration of fluidity thereof. In addition, the wax tends to form films thereof on a carrier, a photoreceptor, and the like, after a long period of use, and therefore the image quality deteriorates. When the toner is a color toner, there is another problem that color reproducibility and transparency deteriorate. When the domains are too small, the wax is too excessively dispersed to impart good releasability to the toner.
Although it is necessary to control the dispersion diameter of the wax domain, there is no appropriate way. In particular, the dispersion diameter of the wax domain of the pulverized toner depends upon the shearing force applied when toner components are melt-kneaded. But it is difficult to apply a proper amount of shearing force to a polyester resin, which is widely used as a binder resin recently, due to its low viscosity. In this case, it is difficult to control the dispersion diameter of the wax domain.
In a pulverized toner, there is another problem that the wax tends to exist at pulverized sections, i.e., the wax tends to exist at the surface of the toner particles.
The wax is softer and has a larger adhesive property than the resin. Therefore, the wax tends to adhere to a photoreceptor and form a film thereof (this phenomena is hereinafter called to as filming problem) when a large amount of the wax is present at the surface of the toner.
In order to produce high definition and high quality images, toners are improved to have a smaller particle diameter and a narrower particle diameter distribution. Since conventional pulverized toner particles have irregular shapes, the toner particles tend to be excessively pulverized when mixed with a carrier in a developing device (when used for a two-component developer), or when contacting a developing roller, a toner supplying roller, a toner layer thickness controlling blade, a friction-charging blade, etc. under stress (when used for a one-component developer). As a result, the resultant image quality deteriorates because ultrafine particles are produced and a fluidizer is buried in the surfaces of the toner particles. Since such an irregular-shaped toner has poor fluidity, there is a problem that the toner needs a large amount of a fluidizer. There is another problem that a toner bottle must be larger because such an irregular-shaped toner cannot effectively fill up the toner bottle, resulting in disturbing downsizing of the apparatus.
A full-color transfer process in which a full-color toner image is transferred from a photoreceptor to a transfer medium or a paper is complicated. On the other hand, a pulverized toner has poor transferability due to its shape. When the pulverized toner is used for the full-color transfer process, the transferred image may have image defects and a large amount of the toner is consumed so as to compensate the image defects.
Therefore, there are demands for improving transferability of the toner to produce high quality images by reducing image defect and to reduce the running cost by reducing the amount of the toner consumed. If the toner has good transferability, the toner particles tend not to remain on the photoreceptor or the transfer medium, and therefore the apparatus does not need a cleaning unit. As a result, the apparatus can be downsized and the manufacturing cost thereof can be reduced. In addition, waste toner particles are not produced. In attempting to overcome the above drawbacks of the irregular-shaped toner, various methods of preparing a spherical toner have been proposed.
For example, in attempting to improve both low-temperature fixability and hot offset resistance of a toner, a technique in which a release agent (e.g., a polyolefin wax) having a low melting point is added to a toner has been proposed.
JP-A's 06-295093, 07-84401, and 09-258471 have disclosed toners including a wax having a specific endothermic peak measured by a differential scanning calorimeter (DSC). However, these toners do not sufficiently satisfy low-temperature fixability, hot offset resistance, and developability.
JP-A's 05-341577, 06-123999, 06-230600, 06-295093, and 06-324514 have disclosed toners including a release agent such as a candelilla wax, a higher fatty acid wax, a higher alcohol wax, natural plant waxes (a carnauba wax, a rice wax), and a montan ester wax. However, these toners do not sufficiently satisfy low-temperature fixability, hot offset resistance, developability (chargeability), and durability. In general, when a release agent having a low-melting point is added to a toner, fluidity of the toner deteriorates, and therefore developability, transferability, chargeability, durability, and preservability thereof also deteriorate.
JP-A's 11-258934, 11-258935, 04-299357, 04-337737, 06-208244, and 07-281478 have disclosed toners including two or more release agents so as to broaden the fixable temperature range (in which hot offset does not occur) thereof. However, these toners have a problem in dispersibility of the wax in the toner.
JP-A 08-166686 discloses a toner including a polyester resin and two offset inhibitors, each of which has an acid value and a different melting point. However, this toner has insufficient developability.
JP-A's 08-328293 and 10-161335 have disclosed toners including wax particles having a specific particle diameter. However, the existential condition and location of the wax particles are undefined, and therefore the toner has insufficient separativeness when fixed.
JP-A 2001-305782 discloses a toner, on the surface of which spherical wax particles are fixed. When the wax particles are present on the surface of the toner, fluidity thereof deteriorates, and therefore developability, transferability, chargeability, durability, and preservability also deteriorate.
JP-A 2002-6541 discloses a toner in which wax particles are locally present near the surface of the toner particle. However, hot offset resistance, preservability, and durability of the toner is not always satisfactory.
Published examined Japanese patent application Nos. (hereinafter referred to as JP-B) 52-3304 and 07-82255 have disclosed pulverized toners including a styrene resin as a binder resin, a polyolefin (such as a low-molecular weight polyethylene and a low-molecular weight polypropylene) as a release agent, and/or a polyolefin resin to which a styrene resin is grafted. Since the styrene resin does not impart low-temperature fixability to the resultant toner, these toners do not respond to a recent demand for energy saving.
JP-A's 2000-75549, 2001-249485, 2003-202698, and 2003-255589 have disclosed toners including a polyester resin which can impart low-temperature fixability to the resultant toner. These toners are pulverized toners which are prepared by melt-kneading toner components, followed by fine pulverization and classification. The shape and the surface structure of the pulverized toner depend on the pulverization property of the materials used and the pulverization condition, and it is difficult to easily control the shape and the surface structure. It is also difficult to narrow the particle diameter distribution because there is a limit to improve the classification ability and the manufacturing cost is raised. It is also difficult for the pulverized toner to have an average particle diameter of not greater than 6 μm considering yield, manufacturability, and cost.
On the other hand, in a toner manufacturing method in which toner components are discharged from a micronozzle, it is easy to form spherical particles having a smaller particle diameter. But there is a problem of nozzle clogging. In particular, when toner components include coarse particles or aggregations of a release agent, nozzle clogging easily occurs.
The present inventors have found that a toner having a nearly monodisperse particle diameter distribution can be prepared by a method including:
dissolving or dispersing toner constituents comprising a resin and a colorant in a solvent, to prepare a toner constituent liquid;
supplying the toner constituent liquid to a retention part configured to retain the toner constituent liquid;
discharging the toner constituent liquid from the retention part to a granulation space through plural holes arranged on the retention part, while exciting the toner constituent liquid by a vibration means in contact with a part of the retention part, so that the discharged columnar toner constituent liquid is constricted to form liquid droplets; and
converting the liquid droplets into solid toner particles.
In this method, the single vibration means entirely excites the retention part having plural holes. Thereby, a uniform vibration is applied to the toner constituent liquid, which is to be discharged from the retention part through plural holes, and an acoustic wave is generated therein. As a result, more than 100 liquid-droplet-forming phenomena can be simultaneously performed by the single vibration part. This method can solve the conventional problems such as hole clogging, poor manufacturability, and poor stability, and is capable of efficiently producing a toner having a narrower particle diameter distribution than ever before. Such a toner has little or no variation in toner properties (such as fluidity and chargeability) among each toner particles.
However, when the toner constituent liquid includes a wax, the holes are easily clogged with the wax, and therefore the resultant toner hardly has a narrow particle diameter distribution.